1. Field of the Invention
The present invention relates generally to the conversion of electric signals from analog to digital form. More specifically, the present invention relates to a digital circuit for translating an analog audio or like analog signal into an equivalent digital signal having a filtered dither signal added thereto which is an artificially created noise signal for reducing noise and distortion.
2. Description of the Prior Art
The pulse code modulation (PCM) or digital processing of analog audio signals has become, or is becoming, the mainstream of high fidelity sound recording and reproduction with the advent and ever increasing commercial acceptance of compact discs. The digital processing of analog signals in the low to high frequency range is also becoming highly useful for analyzing the performance of weapons systems such as guided missiles. For example, the analog data acquired by a guided missile during flight is converted to its digital equivalent and then placed in a digital PCM frame for transmission to a ground station for analysis.
There are, however, some problems left unsolved in the art of digital processing of analog signals. For example, there is a significant difference between the samples of an analog signal and the quantized values of the samples which is generally referred to as "quantization noise". This difference becomes particularly pronounced when the input analog signal is low and there are only a small number of quantization steps, thereby, for example, distorting the reproduced sound as higher harmonics in high fidelity sound systems.
A prior art solution to this quantization noise problem has been the use of dither which is intended to turn the quantization noise into broad band white noise which has little or no effect on reproduced sound or the like and which is generally referred to as "white noise". One system of the prior art adds an analog dither signal, converted from the output from a digital dither generator by a digital to analog converter, to the incoming analog audio or analog data signal. The combined analog data and dither is digitized by an analog to digital converter and then fed to a subtractor circuit. The subtractor is also supplied the digital dither directly from the dither generator. The subtractor circuit then subtracts the dither component from the digital data and dither signal.
A second system of the prior art uses an analog dither generator in combination with a second analog to digital converter for digitizing the analog dither prior to its delivery to the subtractor. In this prior art system, analog dither is added directly to the analog data signal. The resulting dither added data signal is then digitized by the first analog to digital converter and the analog dither is digitized by the second analog to digital converter preparatory to application to the two inputs of the subtractor circuit.
A prior art patent issued to a co-inventor of the present invention, U.S. Pat. No. 5,311,180 entitled "Digital Circuit For The Introduction And Later Removal Of Dither From An Analog Signal" to Gary S. Borgen describes a first pseudo-random number generator which generates digital random noise signals for addition to the digital equivalent signal of an analog audio signal and a second psuedo-random number generator which generates subtractive digital random noise signals for the subsequent removal of dither from the digital equivalent signal prior to its conversion to an analog replica of the analog audio signal.
While partially satisfactory for their intended of purpose of removing noise and distortion from a digitized analog signal, these prior art systems ordinarily leave something to be desired whenever the incoming analog signal is composed of low level sinusoids, often referred to as a low level sinusoidal signal. Low level sinusoidal signals have a spectral content that is highly dependent on and correlated with the frequency of the sinusoids present in the signal. In a spectral analysis of the digital equivalent signal, the quantization noise tends to be centered about the frequency of the signal and its harmonics generating quantization noise which obscures the signal.
With the above and other disadvantages of the prior art the present invention was conceived and one of its objects is to minimize the quantization noise of an incoming analog signal composed of low level sinusoids when the signal is converted to its digital equivalent, while simultaneously minimizing the number of digital bits to accurately represent the analog signal.
It is another object of the present invention to provide a reliable yet relatively simplistic and inexpensive electronics circuit for generating and then adding digital dither bits to an equivalent digital signal.
It is yet a further object of the present invention to provide electronics circuitry which is capable of operation with high resolution and conversion accuracy.
The above and other novel features and advantages of the present invention and the manner of realizing them will become more apparent and the invention will be best understood from a study of the following description and appended claims, with reference to the attached drawings.